Optical gyroscopes are based on the underlying principle of measurement of the Sagnac effect. The latter induces, under the effect of a rotation, a path-delay difference between two electromagnetic signals propagating in opposite directions along a ring-shaped path. This path-delay difference, which is proportional to the angular velocity of the device, may be measured either as a phase difference in the case of an interferometer set-up, or as an eigenfrequency difference between two contra-rotating modes of a ring cavity.
In the first case, it is necessary to use an optical fibre to maximise the length of the interferometer and therefore the sensitivity of the device. An interferometric fibre optic gyroscope (I-FOG) is then spoken of.
In the second case, the difference between the eigenfrequencies of the modes of the cavity may be measured in two ways. The first consists in using an active cavity, i.e. one containing a gain medium, and in measuring the frequency difference between the contra-rotating modes emitted by the cavity. Ring laser gyroscopes or RLGs are then spoken of. The second way consists in using a passive resonant cavity and in probing the eigenfrequencies of the contra-rotating modes using a laser. A passive resonant gyroscope is then spoken of.
The passive resonant gyroscope configuration possesses a certain number of advantages with respect to the other configurations. With respect to the RLG configuration, it in particular avoids the need to use a gaseous gain medium and the high-voltage system of electrodes that is conventionally associated therewith. With respect to the I-FOG configuration, it has the advantage of a much shorter optical path, this decreasing sensitivity to the environment and increasing compactness. Lastly, it employs only standard components. Thus, in particular, there is no need to use a superluminescent source.
However, although these three types of gyroscopes i.e. the I-FOG, RLG and passive resonant gyroscope, have all been demonstrated experimentally, currently, only the first two have found use in industrial applications. One of the things holding back the development of passive resonant gyrometers is the problem of backscattering of the light, which creates coupling between the contra-rotating modes, this creating a nonlinearity in the frequency response and a “blind spot” just as in conventional laser gyros, and degrading the performance of the system.
One solution to the problem of coupling between contra-propagating modes is described in document FR 1302311. This system probes the eigenfrequencies of the contra-rotating modes of a ring cavity while avoiding the problems conventionally created by backscatter, and while simultaneously providing a measurement of cavity length intended to be used to evaluate (and possibly to servocontrol to a constant value) the scale factor of the passive resonant gyroscope thus produced.
The underlying principle of this system is to use three beams at three different frequencies (instead of the two used in conventional gyroscopes). The system includes a ring cavity and a laser the emission of which is divided into three beams of different optical frequencies. By way of example, the cavity may consist of a hollow fibre to limit the Kerr effect. Each frequency is separated from the two other frequencies by a value corresponding to an integer multiple of the free spectral range of the cavity. The free spectral range (FSR) of the cavity is conventionally:
FSR=c/L c being the speed of light and L the optical length of the ring cavity.
The first beam is servocontrolled to one mode of the cavity in one propagation direction and the two others are servocontrolled to two other modes of the cavity corresponding to the opposite propagation direction. It will be noted that it is also possible to invert the servocontrol, i.e. to servocontrol a first eigenfrequency corresponding to a first resonant mode of the cavity to the first frequency of the first optical beam, for example by servocontrolling the length of the cavity.
The frequencies of the three beams are at any given time sufficiently far apart for the effect of coupling between the beams to be negligible.
In the absence of rotation, each beam is servocontrolled to one different eigenfrequency of the cavity, denoted:
f1=N1·c/L for the first beam;
f2=N2·c/L for the second beam; and
f3=N3·c/L for the third beam;
where N1, N2 and N3 are known integer numbers that are all different.
The frequencies must be sufficiently close for the difference between the frequencies of each pair of beams to be compatible with the passband of a photodiode.
In the presence of a rotation, the frequency difference of the two beams propagating in the same direction allows the length of the cavity to be determined, whereas the frequency difference between two contra-rotating beams combined with the information on the length of the cavity allows the angular velocity of the assembly to be determined.
Thus, a gyroscope operating with 3 frequencies comprises means for measuring the frequency difference of the two beams propagating in the same direction, and for measuring the frequency difference between two contra-rotating beams, these two frequency differences combined together allowing the length of the cavity and the angular velocity of the cavity about an axis perpendicular to the cavity to be determined. To simplify the description, these conventional measuring means are not shown in the figures.
Specifically, in the presence of a rotation, the eigenfrequencies are shifted by an amount Ω proportional to the angular velocity, thereby giving:f1=N1·c/L+Ω/2;f2=N2·c/L−Ω/2; andf3=N3·c/L−Ω/2.
At any given time, the length of the cavity may be determined by measuring the frequency difference Δfp between the beams propagating in the same direction, i.e. in the above example Δf2-3:
  L  =                    (                              N                          2              -                                ⁢                      N            3                          )                    Δ        ⁢                                  ⁢                  f                      2            -            3                                ·    C  
The speed of rotation is deduced therefrom by measuring the frequency difference Δfp between two beams propagating in opposite directions, in the above example Δf1-2:
  Ω  =            Δ      ⁢                          ⁢              f                  1          -          2                      -          Δ      ⁢                          ⁢                        f                      2            ⁢                          -                        ⁢            3                          ·                              (                                          N                                  1                  -                                            ⁢                              N                2                                      )                                (                                          N                                  2                  -                                            ⁢                              N                3                                      )                              
The architecture proposed in document FR 1302311 is shown in FIG. 1 with mirrors. The solid lines correspond to optical paths and the dashed lines to electrical connections. The laser L emits a beam that is divided into three beams F′1, F′2 and F′3. To simplify the description, the means for measuring Δfp and Δf1-2 have not been shown.
F′1 is for example injected into the ring optical cavity C of length L in the counterclockwise or CCW direction, whereas the two beams F′2 and F′3 are injected into the cavity in the clockwise or CW direction. The portion of the beams F′2 and F′3 transmitted through the coupler 10 (semi-silvered mirror) passes through the optical fibre and is reflected by the optical coupler 11, then the coupler 10, so as to form the cavity. The portion of the beam F′1 transmitted through the coupler 11 passes through the optical fibre and the coupler 11, and is reflected by the coupler 10 so as to form the cavity.
At resonance, the back-reflected intensity output from the cavity is minimal, and this property is used to servocontrol the frequencies of the three beams to the eigenmodes of the cavity. For example, the beam 101 reflected by the coupler 11 downwards in FIG. 1 is used to servocontrol the frequency of F′1. Said beam corresponds to the coherent superposition of the portion of the beam F′1 directly reflected by 11 and the portion formed by the beams propagating in the cavity in the CCW direction, which result from the superposition of beams having made one, two, three, etc. complete circuits of the cavity in the CCW direction. Likewise, the beams 102 and 103 directed upwards in FIG. 1 are respectively used to servocontrol the frequencies of F′2 and F′3. Said beams correspond to the coherent superposition of the portion of F′2 and F′3 directly reflected by the coupler 10 and the portions of F′2 and F′3 transmitted by this coupler 10, then guided through the cavity in the CW direction, then reflected by 11 and lastly transmitted by 10, corresponding to the superposition of the beams having made one, two, three, etc. complete circuits of the cavity in the CW direction.
The beam F′1 is servocontrolled to an eigenmode of the cavity by controlling the laser L directly using the photodiode PhD1 and a servocontrol device DA′1, which includes an optical portion DA′o1 and an electrical portion DA′e1.
The beams F′2 and F′3 are servocontrolled to eigenmodes of the cavity using the photodiode PhD23 and servocontrol devices DA′2, DA′3, which each include an optical portion (DA′o2, DA′o3) acting directly on the optical frequency, and an electrical portion (DA′e2, DA′e3).
More generally one of the beams has an eigenfrequency maintained at resonance—in the nonlimiting example, the beam F′1 (but it could be one of the other beams)—by direct servocontrol of the laser in the variant Opt1 illustrated in FIG. 2. According to another variant Opt2 illustrated in FIG. 3, the frequency of the beam F′1 is maintained at resonance by directly servocontrolling the length L of the cavity, for example using a piezoelectric modulator.
We will now explain the way in which the laser is directly servocontrolled, such as illustrated in FIG. 2. The beam F′1 passes through a phase modulator PM1 so as to generate sidebands required to obtain a frequency error signal ε1 allowing the frequency to be servocontrolled so as to place (absence of rotation) or keep (presence of rotation) the frequency of the beam F′1 in resonance with the cavity mode in question. This method is based on the technique called the Pound-Drever-Hall technique, named after its inventors, and which is well known to those skilled in the art.
The beam 101 is modulated by a phase modulator PM1, which is placed in the optical portion DA′o1, so as to create sidebands at frequencies separated from the initial frequency f′1 by multiples of the modulation frequency, fm1, applied by the oscillator Os1 via PM1.
This frequency is chosen, if possible, to be higher than the width of the resonance of the cavity (and lower than the free spectral range of the cavity) so that the sidebands are not in resonance with the cavity. To simplify the explanation, only the two first sidebands, separated by ±fm1 from the initial frequency f′1, will be considered. The beam 101 (which therefore has three spectral components at f′1−fm1, f′1 and f′1+fm1) is detected by a photodiode PhD1, the output signal of which is demodulated by the modulating signal applied to PM1 with an adjustment of their respective phases (phase shifter Dph1) requiring the use of an electrical mixer M1. A lowpass filter (not shown) then allows the DC component of the demodulated signal to be isolated, the amplitude ε1 of which is then proportional to the difference between the frequency f′1 of the laser and the resonant frequency of the cavity. Specifically, when the frequency f′1 of the laser and the resonant frequency of the cavity differ slightly, the two sidebands are unchanged (if they are far out of resonance) whereas the phase and amplitude of the beam at the frequency f′1 varies (since it is no longer in resonance). The coherence properties of the three spectral components of 101 then allow these fluctuations (three-beam interference) to be measured as they cause a linear variation in the demodulated signal which may thus be used as a frequency error signal, ε1 cancelling out when the beam F′1 is resonant with a mode of the cavity. Servocontrol is then achieved with this signal, via control electronics ER1, using a conventional servocontrol method, for example, non-limitingly, PI or PID control devices (PID standing for proportional integral derivative, a reference to the three modes of action on the error signal of the control electronics). This type of control, which allows the error signal to converge on a zero value, is well known in automatic control.
Regarding the choice of the modulation frequency to be applied to PM1, if the finesse of the cavity is high, the width of the cavity will be small relative to the free spectral range and it will be possible to choose a modulation frequency that is very high relative to the frequency width of the resonant peaks of the cavity. The optimal situation, corresponding to the preceding explanation, will then be achieved for this servocontrol. In contrast, if the finesse of the cavity is not very high, the modulation frequency will be close to the frequency width of the resonant peaks of the cavity. The sidebands will then be partially modified when the frequency f′1 differs from the resonance and the servocontrol will be less effective.
The servocontrol loop controls the laser for example via the current injected (FIG. 2) so as to set (absence of rotation) or keep (presence of angular rotation Ω) the frequency f′1 of the laser equal to a resonant frequency of the cavity:f′1=N1·c/L+Ω/2
In the embodiment in FIG. 3, it is the length of the cavity that is servocontrolled, the frequency of the laser remaining constant.
Thus, the optical portion DA′o1 of the servocontrol device DA′1 comprises the phase modulator PM1 and the electrical portion DA′e1 connected to the output of the photodetector PhD1 comprises a demodulating portion containing the phase shifter PhD1, the mixer M1, and the oscillator Os1 of frequency fm1, which also supplies PM1; and the control electronics ER1.
An exemplary servocontrol mechanism for controlling the frequencies f2 and f3 respectively of the beams F′2 and F′3 is schematically shown in FIG. 4. The servocontrol device is the same as for F′1. However, as there is only a single laser (or only a single cavity) it is no longer possible to act on these elements. It is therefore necessary to introduce two additional components to achieve two additional degrees of freedom allowing f2 and f3 to be servocontrolled.
Thus, the beam F′2 passes through an acousto-optical modulator AOM2 intended to modify the frequency thereof (alternatively a phase modulator allowing frequency changes to be achieved by serrodyne modulation may be used), then the transmitted portion is injected into the cavity in the CW propagation direction.
In the absence of rotation, the average value of the frequency offset, denoted Δfa in FIG. 4, is chosen equal to a multiple of the free spectral range FSR. To this average value is also added (via the AOM2) a modulation signal intended to generate the sidebands required to obtain the signal allowing this average value to be servocontrolled so as to place (absence of rotation) or keep (presence of rotation) the frequency of the beam F′2 in resonance with the cavity mode in question. The frequency f′2 of the beam F′2 is then servocontrolled via Δfa to an eigenmode of the cavity the frequency of which is offset from the frequency f′1 by a chosen amount, and respects, taking into account a possible rotation at the angular velocity Ω:f′2=(N1+1)·c/L+Ω/2 i.e. Δfa=c/L−Ω/2
To do this, the beam 102 described above is detected by a photodiode PhD23 (which is the same for both beams F′2 and F′3). It is then processed in the same way as explained for the beam F′1 with the same considerations regarding the choice of the frequency of the local oscillator Osc2 (frequency fm2) that modulates AOM2 and serves in the demodulation phase.
An error signal ε2 is thus generated that cancels out when the beam F′2 is resonating with the mode of the cavity.
The procedure is the same with F′3 except that the frequency of the oscillator Osc3 is different from that of the oscillator Osc2 but must meet the same criteria as F′1 and F′2 with respect to the frequency width of the resonant peaks of the cavity and its free spectral range. It is thus possible from the single signal delivered by the photodiode PhD23 to generate two distinct frequency error signals, ε2 and ε3, for F′2 and F′3, respectively.
This signal ε2 is used by the control electronics ER2, for example a PID controller, to control the acousto-optical modulator AOM2 so as to maintain the frequency f′2 of the beam F′2 at resonance with the mode of the cavity. To do this, the aforementioned modulation signal is obtained via the adder S2 and the oscillator Os2, thereby generating sidebands allowing the modulated signal that is detected by the photodiode to be obtained.
Thus, the optical portion DA′o2 of the servocontrol device DA′2 comprises the acousto-optical modulator AOM2 and the electrical portion DA′e2 connected to the output of the photodetector PhD23 comprises the phase shifter Dph2, the mixer M2, the oscillator Os2, the adder S2, and the control electronics ER2.
In this system, the acousto-optical modulators are used both to servocontrol the frequency of the corresponding beam (f′2 or f′3) to an eigenfrequency of the cavity, which frequency is different from the frequency f′1 and offset from f′1 by a chosen amount (corresponding to an integer multiple of the FSR that is different for each beam), and to track in real-time the shift in this eigenfrequency due to the rotation Ω. The acousto-optical modulator must therefore be able to create a frequency offset of at least one free spectral range FSR (at least for example N2=N1+1 and N3=N1+2), this introducing a limit on the minimum length of the cavity.
An AOM is typically limited to an offset of about 1 GHz, namely a cavity of a length of 20 cm (taking in this example a cavity made of optical fibre the refractive index of which is 1.5). A cavity of a clearly smaller length would therefore no longer be compatible. In addition, AOMs are bulky and consume a lot of power (typically the RF powers may be of the order of several watts). To produce a sufficiently long cavity, while maintaining, for reasons of bulk, a diameter of a few centimeters to a few tens of centimeters, the optical fibre of the cavity is looped a number of times.
Thus, the presence of acousto-optical modulators in the 3-frequency system of document FR 1302311 makes it incompatible with a “short” cavity.
However a short-cavity optical gyroscope would have a number of advantages:
decrease in the temperature sensitivity of the fibre achieved by decreasing the number of loops;
compatibility with free-space mirror cavities (a single loop), this having the advantage of suppressing the Kerr effect which is a non-linear effect known to limit the precision of fibre gyroscopes whether they be resonant or not;
compatibility with integrated optics, this technology currently being limited to a single loop. Specifically, to produce a number of loops, it would be necessary to produce lossless crossings or indeed a nonplanar integrated optical circuit so that the path allowing the cavity to be closed could pass below or above to avoid crossings. It will also be noted that acousto-optical modulators are at the present time difficult to produce in integrated-optics technology.
One aim of the present invention is to mitigate the aforementioned drawbacks by providing a 3-frequency resonant gyroscope compatible with a short cavity and/or compatible with optical functionalities produced in integrated-optics technology.